A major consideration in the design of any communication system involves the requirement that any subscriber be able to communicate with any other subscriber in the system. The simplest way to satisfy this requirement is to interconnect every subscriber with every other subscriber using separate transmission lines. However, it is apparent that even in the smallest communications system this solution would involve an economically prohibitive number of interconnections. Accordingly, all commercial communication systems use sophisticated switching devices which enable any subscriber to be connected with any other subscriber, as required.
In communication systems which are based on the transmission and processing of information in the form of electrical signals, sophisticated switching systems are economically viable, even for small "local area" networks (LAN's). However, to date, the technology which has been developed for "switching" optical signals may be economically prohibitive for such small "local area" networks. Accordingly, much effort has been expended developing non-switched couplers which permit communication between any pair of subscribers in a local area optical network.
Two major designs have been suggested for such non-switched couplers in optical communication systems. The first is a "bus" system in which every subscriber signal is placed on a "bus" line which can be accessed at will by every other subscriber. In this manner any subscriber can communicate with any other subscriber. Various algorithms have been designed to increase the efficiency with which any subscriber can access the network and insert his information on the bus.
The second major coupler design is commonly called a "star coupler". In the star coupler every subscriber's output is directly transmitted to every other subscriber in the system. The star coupler divides the power entering any of its input ports equally among its output ports. Different messages can be communicated among the various subscribers by using wavelength, or time division multiplexing.
A simple 2.times.2 star coupler is the well known 3 db coupler--two fibers which share their optical power at a point of proximity. Such structures have been built by etching, grinding and polishing, or fusion techniques. Such 2.times.2 stars can be "ganged" to yield larger n.times.n stars, where n is any arbitrary power of two. (See, for example, M. E. Marhic, Hierarchic and Combinational Star Couplers, Optics Letters, Volume 9, Number 8, pages 369-370 (August 1984)). When multimode waveguides are used, the star coupler may be a glass element with each subscriber's output connected on one side of the element, and each subscriber's access line connected to the other side of the element (see, for example, K. Nosu and R. Watinobe, Slab Waveguide Star Coupler for Multimode Optical Fibers, Electronics Letters, Volume 16, Number 15, pages 608-609 (July 1980)).
A major disadvantage of the star coupler system is associated with the fact that each subscriber's signal must be distributed among all of the other subscribers, whether or not they desire to access the first subscriber's signal. Accordingly, power considerations loom large, since the power of each input is divided among all of the subscribers. When single mode waveguides are used the problem of power loss becomes even more significant, since the design of single mode star couplers is a much more demanding problem. Suggested single mode star couplers involve large numbers of "ganged", "stacked" or "cascaded" directional couplers. However, such couplers are extremely difficult to fabricate and economically prohibitive.